Photophysical/quantum mechanical characterization of the compound 3-benzoxazol-2-il-7-hydroxy-chromen-2-one

Abstract

The photophysical properties of the compound 3-benzoxazol-2-il-7-hydroxy-chromen-2-one were studied for different solvents. This compound, due to a dissociation equilibrium based on the hydroxyl group, results in a neutral and an anionic form in protic solvents. Both forms present high molar absorptivities, around 10 4 dm 3 mol −1 cm −1 related to the S 0→S 1 transition, which suggests for this transition a π→ π ∗ character. Theoretical calculations show that this transition must possess some n→ π ∗ perturbational contribution, principally in the case of the anionic form, where the dipole moment of the S 1 state (8.590 D) is lower than the observed for the ground state (9.374 D). For the neutral molecule, the n→ π ∗ perturbational contribution is due to the ketonic group, whereas for the ionized form, this contribution is due to the phenoxide group. The theoretical prediction of the electronic spectra and microstates shows a good agreement with the experimental data, principally when the solvation is considered. This coumarin shows to be highly fluorescent in different solvents, presenting elevated quantum yields and large Stokes’ shifts. The Stokes’ shift for the neutral form, although the S 1 excited state present a higher dipole moment than the ground state, shows a trend to diminish as the solvent polarity increases, due to efficient solvent–fluorophore interactions in the ground state. The participation of non-radiative processes increases as the anionic form becomes preponderant and with the increase in the polarity of the solvent and its capacity to perform hydrogen bonding. Even at low temperature (77 K), phosphorescence was not detected. Meanwhile, a quantum yield of 0.06 for singlet oxygen generation was measured for the neutral species in chloroform, indicating the possibility of intersystem crossing in a low extent.